Much of the Christian population of the besieged Syrian city of Qusair has abandoned the town after an “ultimatum” from the rebel military chief there, reported Agenzia Fides, the official Vatican news agency.

The ultimatum expired on Thursday, and most of the city’s 10,000 Christians fled Qusair, in the battleground province of Homs. On Sunday, government forces shelled rebel-held cities and villages, killing at least 38 people in the rebellious central district, activists said.

Meanwhile, British Foreign Secretary William Hague said that he could not rule out military intervention in Syria, saying the situation there is beginning to resemble the violence that gripped Bosnia in the 1990s.

Just as Bosnia-Hercegovina’s devastating three-year war was fed by sectarian and religious tensions, Christians in Syria fear an Islamist takeover could result in repression.

Christians represent about 10% of the population, but their status in Syrian conflict zones has become more and more tenuous. Many Christians remain loyal to Assad because his government has been tolerant of religious minorities.

In neighboring Iraq after the U.S.-led invasion in 2003 that ousted Saddam Hussein — who, like Assad, was a secular autocrat, militants in post-Hussein Iraq bombed churches, torched Christian shops and forced hundreds of thousands of Christians to flee to Syria, long regarded as safe for Christians.

“Some mosques in the city have relaunched the message, announcing from the minarets: ‘Christians must leave Quasir,’ ” said the Vatican agency, which has sought to document the parlous plight of Syria’s ancient Christian community.

Qusair has been the site of intense clashes for months between armed rebels and forces loyal to President Bashar Assad. The strategic city is close to the Lebanese border and has been a smuggling hub for arms and medicines destined for rebel forces in Homs, about 15 miles to the northeast, whose large Christian population has mostly fled.

Syrian opposition spokesmen have said repeatedly that Syrian rebels do not target Christians or other minorities and believe in creating a democratic society once Assad is ousted. Leading the rebellion are members of Syria’s Sunni Muslim majority, who have long chafed under the rule of the Assad clan, members of the minority Alawite sect, an offshoot of Shiite Islam.

The Vatican agency cited “sources” who said that extremist Islamist groups in the ranks of the Qusair rebels “consider Christians ‘infidels,’ confiscate goods, commit brief executions and are ready to start a ‘sectarian war.'”